Human cytomegalovirus (HCMV), a member of the herpesvirus family, is the leading viral cause of birth defects and causes significant morbidity and mortality in immunosuppressed individuals. This virus appears to affect the host cell metabolism in ways that mimic processes involved in cell activation, but the molecular basis of these effects and their relationship to viral replication have yet to be elucidated. A major goal of our studies has been to decipher the mechanisms by which HCMV usurps various cellular signaling and regulatory pathways to facilitate its replication. Specific focus has been on how the virus activates the cell to a state that is optimal for DNA replication, and at the same time alters the activity of selected cellular proteins so that viral replication proceeds at the expense of the host. Our studies have shown that the HCMV-mediated effects on the cell cycle are at the level of transcription, subcellular localization of proteins, and protein phosphorylation and degradation. In this renewal application, we propose to continue to investigate the mechanisms by which HCMV exploits the host cell. The approach is to couple in vivo genetic and functional analyses with in vitro biochemical and molecular assays to achieve the following specific aims: 1. Determine the effects of the HCMV infection on the localization of and protein-protein interactions between key cell cycle-related proteins; 2. Determine the function of the cyclin-dependent kinases in the replication of HCMV; and 3. Determine the role of the ubiquitination-proteasome pathway in mediating the effects of the infection on the host cell. These studies are important not only to advance our knowledge of how the complex interactions of viral and host functions relate to viral replication and pathogenesis, but also to help elucidate the general mechanisms that operate to control the cell cycle.